The present invention relates to the field of optical position detecting devices, and more particularly to such devices capable of encoding the position of a light spot generated by a light source, which find applications in 3D vision and object measurement (profilometry), object detection, pattern recognition and target tracking.
Position detectors including components whose relative position or movement is measured are well known. Such detectors have been abundantly described in the literature, as by N. A. Agarkova et al. in xe2x80x9cThe design of a digital electro-optical displacement sensorxe2x80x9d Optical Technology, vol. 38, no. 9, 1971, pp. 532-534; by W. Scholz in xe2x80x9cDetermining tape position with optical markersxe2x80x9d Magnettontechnik, Funkschau, Heft 1, 1976, pp. 42-44; by R. Ogden in xe2x80x9cA high resolution optical shaft encoderxe2x80x9d Journal of IERE, vol. 55, no. 4, 1985, pp. 133138; by U. Griebel et al. in xe2x80x9cA new method to determine accuracy and repeatability of robotsxe2x80x9d Proceedings of the IASTED, 21-26 June 1985, Lugano, Switzerland; by T. Bohme in xe2x80x9cA digital potentiometer for position indication using a microcomputer, Elektroniker, Nr. 8, 1987, pp. 86-88; by D. Varshneya et al. in xe2x80x9cApplications of time and wavelength division multiplexing to digital optical code platesxe2x80x9d SPIE, vol. 838, 1987, pp. 210-213; by P. Auvert et al. in xe2x80x9cMonolithic optical position encoder with on-chip photodiodesxe2x80x9d IEEE Journal of Solid-State Circuits, vol. 23, no. 2, 1988, pp. 465-473; and by A. Kwa et al. in xe2x80x9cOptical angular displacement sensor with high resolution integrated in siliconxe2x80x9d Sensors and Actuators A, vol. 32, 1992, pp. 591-597. Examples of such moving part-based position detectors are also disclosed in patent documents, namely in U.S. Pat. No. 3,500,055 issued on Mar. 10, 1970 to Russell et al.; in U.S. Pat. No. 3,702,471 issued on Nov. 7, 1972 to Kennedy et al.; in U.S. Pat. No. 30 4,180,704 issued on Dec. 25, 1979 to Pettit; in U.S. Pat. No. 4,388,613 issued on Jun. 14, 1983 to Rush et al.; in U.S. Pat. No. 4,405,238 issued on Sep. 20, 1983 to Grobman et al.; in U.S. Pat. No. 4,971,442 issued on Nov. 20, 1999 to Okutani et al.; in U.S. Pat. No. 4,948,968 issued on Aug. 14, 1990 to Matsui; in U.S. Pat. No. 5,497,226 issued on Mar. 5, 1996 to Sullivan; in U.S. Pat. No. 6,080,990 issued to Watanabe et al. on Jun. 27, 2000; in Deutche Democratic Republic Patent Specification no. 283001, 1985, naming Rossler et al. as co-inventors; and in European Patent Specification published under no. 490206 on Jun. 17, 1992, naming Durana et al. as co-inventors.
In many fields there is a need for finding the position of a light spot or peak of a relatively small size, wherein known position detectors involving lo relative movement between detector components cannot be used. Some applications can be found in artificial vision where a light beam is scanned over a surface or a volume and the position of the spot is indicative of either the position or the thickness of an object. In pattern recognition, applications can be found in optical processing (e.g. optical correlator) where the optical device transposes the presence of an object into a sharp light peak. In other applications such as in the fields of object detecting and target tracking, a light source or the illuminated part of an object is imaged as a moving small spot whose position must be rapidly detected.
Existing technologies for light spot position detection generally use three different approaches.
According to a first one, a scene containing the luminous spot or peak is acquired with a video camera. The image is then processed by a computer to detect the maximum intensity value to find the corresponding position. However technologies using this approach are generally characterized by limitations related to processing speed, system complexity and cost. Speed limitations are due to the acquisition process with the video camera and to the data processing performed by the computer. For conventional existing cameras, the acquisition process typically takes {fraction (1/30)} sec. Although high-speed cameras with image acquisition frequency around a few kHz are available, they may not be suitable for high rate scanning or fast moving spot applications, such as object tracking. Furthermore, even using a high-performance, high-speed camera, the processing time necessary to detect the maximum intensity value from raw image signals to find the corresponding position of the light spot may still significantly limit detection performance of the system. Such system requiring a high performance camera with a computer running particular analysis software or equivalent high level processing instrumentation, it may be complex to program, calibrate and/or operate, as well as expensive. Such a video position sensor is proposed by E. Lanz in xe2x80x9cElectro-optic sensor for position detection and object identificationxe2x80x9d Automation Technology with Microprocessors, Interkawa congress 1977, pp. 95-106, which sensor is based on electronic sequential processing of a two-dimensional video signal.
Another way to proceed is to use position-sensitive electronic devices. A photodiode-based position measuring system is taught by H. Janocha in xe2x80x9cUniversally usable position measuring system with analog displaying position sensitive photodiodesxe2x80x9d Technisches Messeen tm, Heft 11, 1979, pp. 415-420. Such system combines photodiodes that are sensitive to the two-dimensional position of a light source, with an electronic processing circuit generating a position indicative analog signal. Such system is disadvantageous because additional encoding is required to further process the position signal with a digital computer, therefore increasing processing time. A one-dimensional position detector requiring signal pre-processing to generate a digital output is also described by Smith et al. in xe2x80x9cAn integrated linear position sensitive detector with digital outputxe2x80x9d Transducers 1991, Digest of Technical Papers, 24-27 June 1991, San Francisco, pp. 719-722. A coded aperture light detector for use with a three-dimensional camera is disclosed in U.S. Pat. No. 4,830,485 issued on May 16, 1989 to Penney et al., which detector provides a direct digital representation of a range or height position of a reflecting surface of an object. A light spot reflected from the surface is optically spread into a line segment so it can be shared among a number of light detection channels coupled through a segmented fiber optic bundle to a corresponding number of photo-multipliers or solid state detectors. Although not requiring pre-processing, the proposed detector is significantly limited in its resolution due to the mechanical coupling required between each fiber optic of the bundle and each corresponding channel of the coded aperture. Furthermore, several rows of channels being required on the coded aperture to generate a multi digit signal, such detector would be hardly practicable for bi-dimensional spot positioning. Another position-sensitive electronic device is disclosed by Yamamoto et al. in xe2x80x9cNew Structure of Two-dimensional Position Sensitive Semiconductor Detector and Applicationxe2x80x9d IEEE Trans. Nucl. Sci., NS-32, 1985, pp 438-442. The voltage output of such semiconductor device depends on the position of the centroid of the illumination pattern projected on it. This device has the potential to be very fast (around 100 kHz) and is less complex than the camera/processing computer system. However, in computing the mass center of the peak, this device is more sensitive to noise coming either from background of from other sources showing lower intensity. Moreover, resolution and speed are affected by the intensity of the light peak.
The third detection scheme is based on the use of diffractive devices such as diffraction gratings. One-dimensional and two-dimensional light spot position detecting devices are disclosed in U.S. Pat. No. 4,025,197 to Thompson. The one dimensional detecting device disclosed uses a first linear grating disposed before the focal point of an incident laser beam, the modulated emerging beam being directed to a second linear grating disposed in parallel relationship with the first grating. The device include an optical detector coupled to an electronic circuit for generating displacement or position data from the detected diffraction pattern after passing through both linear gratings. The two-dimensional position detecting device as taught by Thompson uses a first X-Y grating formed by two crossing sets of parallel lines and disposed before the focal point of an incident laser beam, a beam splitter receiving the laser beam modulated by the first X-Y grating to produce two separate beams that are respectively directed to an X grating and an Y grating, the former having its lines disposed optically parallel to one of the two sets of parallel lines on the X-Y grating, the latter having its lines disposed optically parallel to the other of the two sets of parallel lines on the X-Y grating. The X grating is followed by a first detector provided with a first electronic circuit for generating displacement or position data from the detected diffraction pattern after passing through the X-Y grating and X grating. In a same manner, the Y grating is followed by a first detector provided with a first electronic circuit for generating displacement or position data from the detected diffraction pattern after passing through the X-Y grating and X grating. However, this device using many optical elements, it cannot be easily built as a compact package, as required in many applications. Another diffractive device is taught by Bergeron et al. in xe2x80x9cDamman-grating-based optodigital position converterxe2x80x9d Optics Letters, vol. 20, 1995, pp. 1895-1897. Using binary patterns and replicated images, the disclosed position converter can be extremely fast (1-100 MHz). However, this converter using also many optical elements, it cannot be easily built as a compact package. Furthermore, its optical elements requiring precise alignment, its use may be laborious and limited to highly skilled technicians.
It is a main object of the present invention to provide a simple, optical device for encoding the position of a light spot.
It is another object of the invention to provide a light spot position encoding device that integrates processing, compression and conversion of data entirely optically, thus avoiding the use of electronic hardware for processing large amount of data;
It is another object of the invention to provide a light spot position encoding device and method exhibiting parallel optical processing capabilities to provide high speed position encoding, without requiring generation of a replicated image of the scene containing the light spot.
It is a further object of the invention to provide a light spot position encoding optical device that is of a compact, light weight design and comprising no moving part.
It is a still further object of the invention to provide light spot position encoding devices and methods capable of encoding position with respect to one-dimensional, two-dimensional or three dimensional coordinates reference system.
The invention proposed herein provides a simple optical device and method of detecting the position of a light spot generated by any light source of either light generating or light reflecting type, directly in a binary or other encoded format at very high speed.
According to the above main object, from a broad aspect of the present invention there is provided an optical device for encoding the position of a light spot image formed at an input image plane, the device comprising a diffractive optical element disposed within the input image plane and including an array of diffractive cells each being disposed at a predetermined position with respect to a predetermined reference point on the diffractive optical element, each said cells being capable of generating a unique optical diffraction pattern when illuminated, at least one of the cells being positioned to receive the light spot input image generating its unique optical diffraction pattern accordingly at an output image plane. The device further comprises one or more optical detectors disposed at the output image plane and responsive to the unique optical diffraction pattern to generate one or more encoded signals indicative of the position of the light spot image with respect to the reference point.
From a further broad aspect of the invention, there is provided an optical device for encoding the position of a light peak generated by an optical processor receiving an image to be processed as generated by an imaging device illuminated by a laser source, said processor comprising first Fourier transform means for performing the Fourier transform of the input image to generate a corresponding transformed input image in the spatial frequency domain within an area defined by a Fourier transform filter plane, optical mask means disposed within said area, said second optical mask means implementing a filter mask function to generate a combined image in the spatial domain, and second Fourier transform means for performing the inverse Fourier transform of the combined image to generate the light peak at a peak image plane. The optical device comprises a diffractive optical element disposed within the peak image plane and including an array of diffractive cells each being disposed at a predetermined position with respect to a predetermined reference point on the diffractive optical element, each said cells being capable of generating a unique optical diffraction pattern when illuminated, at least one of said cells being positioned to receive the light peak and generating its unique optical diffraction pattern accordingly at an output image plane. The device further comprises one or more optical detectors disposed at the output image plane and responsive to the optical diffraction pattern to generate one or more encoded signals indicative of the position of said light peak with respect to the reference point.
According to a still further broad aspect of the invention, there is provided a method of encoding the position of a light spot, said method comprising the steps of: a) forming an image of the light spot at a corresponding position within an input image plane and with respect to a predetermined reference point of said plane; b) generating a unique optical diffraction pattern associated with said corresponding position at an output image plane; and c) detecting the unique optical diffraction pattern to generate one or more encoded signals indicative of the position of the light spot image with respect to the reference point.
Conveniently, said detecting step c) includes separately detecting complementary portions of the unique optical diffraction pattern to generate corresponding ones of said encoded signals.